Secure Group Chats With MLS You Can Ship: Keys, Servers, and Real‑World Rollouts

Why Group Chats Need a Fresh Start

Group messaging looks simple until you try to do it securely, at scale, on phones that skip network coverage for hours at a time. Traditional end‑to‑end encryption protocols were great at one‑to‑one chats. But once a group grows and members join, leave, or add devices, those designs bend—and often break—under churn.

Message Layer Security (MLS) is a new IETF standard that makes large, dynamic groups practical and safe. It brings strong security guarantees—confidentiality, authentication, forward secrecy, and post‑compromise security—while keeping updates efficient even as your group size climbs. If you are building chat into a consumer app, a workplace tool, or a community platform, MLS is the first protocol that treats group encryption as the default case instead of an afterthought.

This article focuses on what it takes to ship MLS in real products. You will learn how the protocol works, how to design your servers and clients, what to watch for in UX, how to measure performance, and how to plan smooth rollouts. No fuss, no mystique—just the pieces you need.

MLS in Plain Language

MLS secures group messages with a shared secret that only the current members know. Unlike pairwise schemes that encrypt N copies for N people, MLS uses a tree of keys and performs updates efficiently, in roughly log(N) time. The method is called TreeKEM (tree‑based key exchange).

What Makes MLS Different

Group‑native design: It was built for dynamic groups, not retrofitted from one‑to‑one chat.
Asynchrony friendly: Devices can go offline, then rejoin and catch up without a round‑trip to every member.
Scales cleanly: Adding, removing, or updating a member’s keys touches only a path in the tree, not the entire group.
Strong guarantees: You get confidentiality, sender authentication, forward secrecy, and post‑compromise security.

Core Building Blocks

Three elements do the heavy lifting:

Credential and signature keys: Each device has an identity (a credential) and a long‑term signing key. Messages and membership changes are signed so everyone can verify who did what.
HPKE for key exchange: Hybrid Public Key Encryption (HPKE) secures the “welcome” secrets and path updates that refresh group keys.
TreeKEM state: Each member tracks a tree of secrets. When someone joins, leaves, or updates, the tree changes and a new epoch begins.

Every change—add, update, remove—produces a new epoch and a fresh application key. Members process the change, advance to the new epoch, and continue chatting. The server delivers messages but never sees the plaintext.

Where MLS Fits in Your Architecture

MLS is a protocol that runs on your clients. The server is a delivery platform and a directory for public keys. There are three major surfaces to design:

1) Delivery Service (Server)

Stores key packages: These are pre‑published, authenticated public keys that other clients use to add a device to a group.
Relays messages: Distinguishes control messages (handshakes that change the tree) from application messages (chat content).
Enforces fairness: Throttling, anti‑spam, and storage policies. The server doesn’t need to trust the content to do this.

2) Clients (Apps)

Own the MLS state: They maintain the tree, process updates, and encrypt/decrypt messages and handshakes.
Handle membership UX: Show who joined, left, and when the group key rotated.
Manage devices: Users can add or remove devices, each with their own key package and credential.

3) Identity and Discovery

Credential issuance: How does a device authenticate? Options include your own CA, WebAuthn‑bound credentials, or conventional account logins with device registration.
Key transparency (optional but wise): Publish a verifiable log of user device keys to detect misissued credentials.

Choosing Cipher Suites and Libraries

To get started, pick a standard cipher suite that’s widely implemented and well‑reviewed. A common choice today is MLS10_128_DHKEMX25519_AES128GCM_SHA256_Ed25519. It’s fast, audited, and available across platforms. If you want room for future post‑quantum upgrades, plan for hybrid HPKE (e.g., X25519+Kyber) when your libraries support it.

Production‑grade Libraries

OpenMLS (Rust): A reference‑grade implementation that compiles to multiple targets and is designed for safety.
mlspp (C++): A performant, embeddable library suitable for native mobile and desktop integrations.

Wrap these in a thin platform layer that abstracts storage, networking, logging, and timers. Keep the MLS logic isolated; it will make audits and upgrades easier.

Designing the Server Side Without Seeing Messages

Your server delivers end‑to‑end encrypted traffic, so think like a mail room, not a security checkpoint. It needs to provide dependable delivery and sane quotas, while staying agnostic to content.

Core Server Responsibilities

Key package directory: Store and serve authenticated key packages per user device. Include expiration times and rotation policies.
Outbox/inbox queues: Buffer handshake and application messages per recipient device. Use per‑queue quotas to avoid spam or DoS.
Push notifications (optional): Wake sleeping mobile clients whenever new items arrive.
Basic metadata indexing: Timestamps, sender IDs (public), group IDs, and message types (handshake vs application) for delivery and diagnostics.

Operational Controls

Rate limits: Per device and per group.
Storage limits: Sliding windows for offline clients; explicit backpressure signals when inboxes overfill.
Audit logging (metadata): Don’t log plaintext. Do log delivery attempts, queue sizes, and suspicious access patterns.

The server can be simple and fast. You don’t need to implement crypto on the server, which makes scaling much easier.

Client State, Storage, and Sync

On the client, MLS state is precious. If you lose it, you fall out of sync with the group. Treat MLS state like a small database you must handle carefully.

Persisting Group State

Snapshot after each epoch: Store the current tree, the group context, and the epoch number atomically.
Separate key storage: Use secure enclaves/keystores when available for long‑term signature keys. Application epoch keys can be derived and cached transiently.
Crash‑safe writes: Use write‑ahead logs or double‑buffering so a power loss can’t corrupt the only copy.

Handling Out‑of‑Order and Forks

Out‑of‑order: Queue messages by epoch and process in order. If you see a future epoch, pause app messages until handshakes catch up.
Fork detection: The MLS tree hash authenticates the state. If two different paths appear for the same epoch, raise an alert and require user review.

Multi‑Device Reality

Each device is a first‑class member with its own credential and key package. When a user adds a device, your client should:

Publish the new device’s key package to the directory.
Issue a membership change to add the device to each selected group, or rely on the new device to be added when it first shows up.
Show a clear UI that a new device is now part of the user’s identity and can read group messages going forward.

Membership Changes and Human‑Centric UX

Some of the most common questions about secure chat are human questions:

Who is in this group right now?
When did they join?
Are my messages visible to them?
What happens if someone leaves?

Make Group Changes Obvious

Readable events: “Taylor added Casey’s laptop” and “The group key was refreshed.”
History boundaries: When a new member joins, mark the timeline where they started having access.
Removal semantics: On removal, rotate the group key immediately. New messages are unreadable to the removed member.

Trust Indicators

Device badges: Show how many devices a user has and which ones are active.
Key verification: Empower users to compare a short safety code or check a transparency log for each device.
Clear risk language: If you detect a fork or suspicious device addition, warn with concrete steps: “Review devices” or “Lock group until verified.”

Performance: What It Costs and How to Keep It Snappy

MLS is efficient, but not free. Here’s what to plan for:

Group Size and Update Cost

Updates cost ~log(N): Adding or removing a member updates a path from the leaf to the root. For a hundred members, that’s still small.
Batch churn: When multiple devices join in quick succession, coalesce updates to avoid bursty key rotations.

Bandwidth and Battery

Handshake overhead: Handshake messages are heavier than app messages. Cache “ratchet tree” extensions when allowed to avoid asking the server repeatedly.
Mobile wakes: Use push notifications with message counts, then fetch in batches to reduce radio on‑time.
Sleep‑friendly timers: Don’t assume low clock drift. Rely on server timestamps for ordering, then verify by MLS epoch.

Crypto Choices

X25519/Ed25519: Fast on mobile CPUs. AES‑GCM or ChaCha20‑Poly1305 both perform well, depending on hardware.
Hybrid readiness: Design your suite negotiation so a future Kyber hybrid HPKE can be rolled out without breaking clients.

Recovery, Backups, and When Things Go Wrong

Users lose phones. Apps get uninstalled. Your design should make recovery safe and understandable.

Crash and Restore

Local backup: Encrypted, device‑bound backups tied to the user’s platform keystore work well. They restore the most recent MLS state.
Fresh join fallback: If state is lost, the device can rejoin the group as a new member. Make it clear that past history won’t be re‑decrypted.

Account Recovery

Don’t reissue unknown devices silently: Recovery should produce a new device with a visible event, not a hidden clone with past access.
Use login approvals: Require an existing device to approve new devices when practical. If none exist, rely on out‑of‑band checks.

Threats, Limits, and How to Raise the Bar

MLS provides strong cryptography, but your security posture is more than math.

What MLS Protects

Confidentiality: Only current members can decrypt current messages.
Authentication and integrity: Every message and membership change is signed by a known device.
Forward secrecy: Past messages stay safe if a device key leaks today.
Post‑compromise security: After you rotate keys, future messages are safe even if an attacker briefly had access.

What MLS Doesn’t Solve Alone

Metadata privacy: The server sees who talks to whom and when. Use basic minimization and consider mix or delay strategies only if your threat model requires it.
Endpoint risks: Compromised devices can read messages. Encourage lock screens, OS updates, and anti‑malware hygiene.
Screenshots and cameras: Users can always exfiltrate content. Set expectations and—if needed—add visible watermarks for sensitive rooms.

Key Transparency and Device Audits

To detect malicious server behavior or rogue device additions, add a key transparency component. Publish device credentials in an append‑only log and let clients verify consistency. Pair that with in‑app device lists users can review and revoke.

Compliance Without Backdoors

Some deployments need archiving for legal reasons. You can do this without weakening security:

Designate a compliance device: Add a read‑only “archive bot” as a regular MLS member whose private key is controlled by your compliance hardware (e.g., an HSM). Everyone can see it in the device list.
User awareness: Clearly label rooms that have an archive participant so people know their messages are being journaled.
No shared master keys: Avoid server‑side decrypt‑on‑demand. That’s a backdoor and defeats end‑to‑end security.

Rolling Out MLS in an Existing App

If you already ship a chat feature with a different encryption scheme, you can migrate in stages.

Phased Deployment

New conversations default to MLS: Keep existing rooms on the old protocol for continuity, but nudge users to create new MLS rooms.
Bridge with read‑only bots if needed: For a while, mirror messages into MLS rooms to ease the shift.
Retire legacy rooms: Once most users have moved, freeze legacy rooms to read‑only and finally archive them.

Versioning and Capabilities

Advertise supported suites: Clients publish what cipher suites and extensions they support. Match groups accordingly.
Hard rules for critical features: If transparency or specific extensions are required, refuse to add devices that don’t support them.

Testing, Interop, and Ongoing Assurance

MLS is a state machine with many edges. Treat it like you would a database engine: fuzz it, fault it, and watch it behave under stress.

Tests That Matter

Interoperability vectors: Run official test vectors from the spec and cross‑test with multiple implementations (e.g., OpenMLS and mlspp).
Chaos queues: Randomly drop, duplicate, delay, and reorder handshake messages. Clients should converge on the same epoch and tree hash.
Large‑N churn: Simulate hundreds of members with frequent joins/leaves to measure update time and bandwidth.
Battery profiling: On mobile, measure wakeups, radio time, and crypto CPU time for busy groups.

Security Reviews

Protocol assumptions: Verify you didn’t weaken authentication by mixing identities across accounts.
Credential lifecycle: Keys must rotate, expire, and revoke cleanly—both for devices and users.
Secure storage: Confirm long‑term keys never leave hardware keystores where available.

Post‑Quantum Planning Without Panic

You don’t need post‑quantum cryptography today for everyday chat, but you should prepare for it. MLS is designed to evolve. When your library supports hybrid HPKE (e.g., X25519 + Kyber‑768), you can negotiate a hybrid suite, preserving interoperability with modern devices while setting a path for future resilience.

Key point: Don’t roll your own. Wait for standardized hybrid suites in MLS and HPKE, then migrate with capability negotiation and clear release notes.

Practical Checklists You Can Adopt

Before You Launch

Pick a cipher suite supported on all target platforms.
Implement a key package directory with rotation and expiry.
Instrument delivery queues with per‑device and per‑group limits.
Design membership UI for joins, leaves, device adds, and key refreshes.
Build a recovery flow that creates a new device rather than silently cloning old keys.
Run interop vectors and chaos queue tests in CI.

After You Launch

Monitor epoch convergence rates and fork alarms.
Track average handshake size and time‑to‑decrypt on mobile.
Rotate device credentials regularly and expire stale key packages.
Publish a transparency log and expose device lists for user verification.
Plan staged rollout for hybrid PQ suites when mature.

Common Pitfalls and How to Avoid Them

Skipping device UI: If users can’t see and manage devices, they won’t trust the system. Always show device count and recent changes.
Server “helpful” decryption: Resist any design that decrypts on the server, even for diagnostics. Build client‑side tracing instead.
Unbounded inboxes: Offline devices can become sinkholes. Enforce reasonable caps and per‑device limits.
One‑shot state serialize: Single‑file state saves can corrupt on crash. Use atomic snapshots with checksums.
Opaque errors: Epoch mismatches and invalid signatures should produce meaningful user feedback and repair options.

A Note on Federation

MLS doesn’t force you to federate, but it doesn’t block you either. If you want separate domains to communicate, agree on:

Credential trust roots: Whose CA signs device credentials?
Discovery endpoints: Where to fetch key packages for foreign accounts.
Delivery policies: Sane limits, retries, and misbehavior handling between servers.

Start with simple bilateral trust, then layer on monitoring and transparency logs spanning domains.

Bringing It All Together

MLS turns secure group chat into a tractable engineering problem. The protocol gives you a solid foundation, but success comes down to craft: keeping servers simple, managing client state carefully, making membership changes clear, and measuring real‑world performance. If you ship those basics well, your users will get what they need—private conversations that scale with their communities and teams—without constant ceremony or fear.

Summary:

MLS is a group‑native, end‑to‑end encryption protocol that scales with churn and size.
Your server relays messages and stores key packages; clients own crypto and MLS state.
Pick well‑supported cipher suites and proven libraries like OpenMLS or mlspp.
Design clear UX for joins, leaves, device additions, and key refreshes.
Persist client state atomically; handle out‑of‑order and fork scenarios gracefully.
Tune performance with batch updates, efficient push, and mobile‑friendly crypto.
Handle recovery via new devices, not silent key cloning; consider key transparency.
Add compliance with a visible archive device, not server decryption.
Test with interop vectors, chaos queues, and large‑N churn; monitor epoch health in prod.
Plan for hybrid post‑quantum suites when standards and libraries are ready.